Two main subtypes are unambiguously recognized: GSD type Ia (GSDIa) due to a defect of the catalytic unit G6Pase-alpha (or G6PC), and GSD type Ib (GSDIb) due to a defect of the glucose-6-phosphate translocase (or G6PT) [1,2]. The gene discussed is G6PC1; the disease is glycogen storage disease Ib.